1. Field of the Invention
This invention relates to an ultrasonic Doppler blood flow velocity detection apparatus.
2. Description of the Prior Art
An ultrasonic Doppler blood flow velocity detection apparatus is used for detecting blood flow velocity which can be used in diagnosis. There are many types of Doppler blood flow detection apparatus utilizing the Doppler effect through reflection of ultrasonic waves. The ultrasonic Doppler blood flow velocity detector can select a portion where blood flow is to be detected with respect to distance and direction.
The most popular ultrasonic Doppler blood flow velocity detection apparatus detects blood flow velocity as follows:
Such an ultrasonic Doppler blood flow velocity detection apparatus transmits an ultrasonic-wave pulse whose center frequency is "f", at a predetermined interval into the human body by a transducer; then it receives a reflected signal, i.e., an echo signal, from a moving reflective object, such as a blood corpuscle; and detects the amount of phase shift of the echo signal, i.e., Doppler shift. An output signal of the phase shift amount, i.e. Doppler signal, indicates blood flow velocity.
In this conventional Doppler blood flow velocity detection apparatus, the relation between a shift frequency fd of a Doppler signal and blood flow velocity V is given by: EQU fd=(2V/c)fc.multidot.cos.theta. (1)
where fc is a frequency of ultrasonic waves; "c" is a sound velocity in the human body; and .theta. is an angle made between the ultrasonic transmitting direction and the direction of blood flow.
Structure described in "MEDICAL ELECTRONICS" Vol 11, No. 3, 65, 1976 is generally known which utilizes the above-mentioned principle. Hereinbelow will be described the above-mentioned prior art ultrasonic Doppler blood flow velocity meter generally with reference to FIG. 12 drawings.
FIG. 12 is a block diagram showing structure of a prior art ultrasonic Doppler blood flow velocity detecting apparatus. In FIG. 12, numeral 31 is an ultrasonic-wave probe generally comprising a piezoelectric device for transmitting ultrasonic waves into a human body 32 and for receiving an echo signal from the human body 32, which is used for obtaining blood-flow information of the blood flowing in vessel 33 at velocity V. Numeral 34 is a driving circuit for producing transmission signal Tx for generating ultrasonic wave by the ultrasonic-wave probe 31. Numeral 35 is a Doppler signal detecting circuit for quadrature-detecting the echo signal Rx received by the ultrasonic-wave probe 31 to output quadrature Doppler signals Vdx and Vdy. Numerals 36 and 37 are A/D converters respectively for A/D converting the quadrature Doppler signals Vdx and Vdy into quadrature Doppler signals Ddx and Ddy of the form of numerical value. Numeral 38 is frequency analyzer for frequency analyzing the quadrature Doppler signals Ddx and Ddy through a calculating method, for example, fast Fourier transformation method (FFT). Numeral 39 is a display for indicating frequency of Doppler signals in the form of sonogram. Numeral 40 is a control circuit for controlling the drive circuit 34, Doppler signal detection circuit 35, and A/D converters 36 and 37.
Hereinbelow will be described operation of the above-mentioned prior art apparatus.
The drive circuit 34 generates transmission signal Tx in response to a control signal from the signal generation circuit 40, which transmits ultrasonic waves into the human body 32 through the ultrasonic-wave probe 31. The ultrasonic waves propagating through the human body 32 are reflected by the blood and return to the piezoelectric device of the ultrasonic probe 31 with a delay time proportional to depth of the portion where the ultrasonic waves are reflected. If the reflective object moves, the echo signal is subjected to Doppler effect with Doppler shift frequency fd in accordance with the relation of the above-mentioned Eq. (1). Frequency of the echo signal received by the ultrasonic-wave probe 31 is fc+fd by Doppler effect. The echo signal is quadrature-detected with quadrature reference signals R of frequency fc by the Doppler signal detecting circuit 35 to output Doppler signals Vdx and Vdy. These Doppler signals Vdx and Vdy have quadrature relation to each other and are of frequency fd which is obtained from difference between frequency fc+fd of the echo signal Ec and frequency fc of the quadrature reference signal. The A/D converters 36 and 37 sample in response to a sampling clock signal Sp sent from the control circuit 40 and A/D-converts the Doppler signals Vdx and Vdy to send digital values Ddx and Ddy to the frequency analyzer 38. The frequency analyzer 38 performs frequency of plural values of equally spaced points of the Doppler signals Ddx and Ddy through FFT method, etc. and indicates the result on display 39, such as television display to provide diagnosis data.
However, in the above-mentioned prior art ultrasonic Doppler blood flow velocity detection apparatus, there is a problem as follows:
Recently, attempts were made to use the ultrasonic Doppler blood flow velocity detection apparatus for diagnosis of blood flow whose velocity is relatively low such as that of the vessel of the abdomen or capillary. An, extremely high signal to raise (SN) ratio is required for circuits used in the above-mentioned apparatus, because these blood flows are very thin compared with arteries and veins of the heart or its peripheral portion so that magnitude of Doppler signal obtained therefrom is extremely small. Such apparatus cannot be realized. Moreover, when low velocity blood flow is detected, in the frequency analyzer 8 using the FFT method, etc. the number of analyzing points of the data is Z.sup.n (n=1, 2, 3, . . . ) and generally constant. Thus, resolution of low frequency components is increased by lowering sampling frequency of Doppler signal. Lowering the sampling frequency, decreases the Nyquist frequency fnq so that overlap occurs if Doppler frequency fd exceeds the Nyquist frequency fnq; Therefore, when attempting to observe flow components of low velocity among a variety of velocity components of a blood flow, the observation of the low velocity components is disturbed due to overlap caused by the high velocity components.